Glycosphingolipids are important components of plasma membranes and such lipids play an important role in numerous biological processes, including cellular recognition processes and numerous signal transduction pathways. A vast number of naturally occurring glycosphingolipids are characterized by the presence of D-erythro-sphingosine, the primary hydroxyl group of which is glycosylated while the amino function is acylated by a fatty acid, thus constituting a ceramide unit.
In recent years, glycosphingolipids are gaining interest for cosmetic, pharmaceutical and therapeutic applications.
Although various lipids have been found to play a role in stratum corneum homeostasis, ceramides are assumed to be the main lipid components within the intercellular lipid membranes for holding the corneocytes together, and ceramides tend to be essential components in building and maintaining the barrier function of the stratum corneum. The observation that topical application of ceramides improves the barrier function of the stratum corneum as well as the moisture retaining properties of the skin, amongst others, account for this increasing interest. Besides improving the function of the stratum corneum, topical application of ceramides has been found to assist in repairing damaged epidermal barrier function. Following these observations, a wide variety of cosmetic and pharmaceutical products containing sphingoid bases, ceramides and glycosphingolipids have been developed.
Initially, sphingolipids for cosmetic and pharmaceutical applications were obtained by extraction of animal tissue. However, the thus obtained sphingolipids are a heterogenous mixture of a plurality of structurally different sphingolipids, which are potentially unsafe due to the possible presence of hazardous micro-organisms. Besides this, extraction from tissue is a laborious and expensive method, with a low yield. In order to meet the growing demand for sphingolipids, numerous attempts have been made to develop synthetic pathways for producing sphingolipids and their sphingoid base building blocks.
A structural unit common to ceramide 1, 1a, 2, 4, 5 is D-erythro-sphingosine (see formula 1 below). Examples of synthetic pathways to sphingosine are disclosed, for example, in Shapiro D. et al, J. Am. Chem. Soc. 1954, 76, 5897-5895, Lees W. J. Tetrahedron Letters, 2001, 42, 5845-5847, Compostella et al., Tetrahedron Letters, 2002, 58, 4425-4428.
In P. M. Koskinen, Synthesis 1998, 8, 1075-1091 several chemical synthetic routes for the synthesis of sphingosine are disclosed. One of the main issues in developing a suitable chemical process appears to be the problem of obtaining sphingosine in its naturally active configuration, i.e., D-erythro-sphingosine. To achieve this, synthesis was started from chiral starting products. Chirality was introduced in subsequent reaction steps through catalysis, or a chiral auxiliary was added in case non-chiral compounds were used as starting compounds. However, all these chemical synthesis routes have in common that the routes involve a large number of reaction steps, which often provide a low yield, a low trans/cis selectivity of the olefinic bond and a low stereo specificity.
In R. Wil et al, Tetrahedron Asymmetry, 1994, 2195-2208 a method is disclosed for a stereoselective synthesis of D-erythro-sphingosine, starting from D-galactose. This prior art method however comprises a large number of subsequent reaction steps, several of them giving incomplete conversion and a low selectivity to the desired product. In this prior art chemical synthesis method, the key Grignard reaction for elongating 2,4-O-benzylidene-D-threose into the desired D-arabino configurated sphingosine precursor gave poor diastereo selectivity. Mesylation of this precursor favors the C-2 OH group, which is subsequently subjected to azide substitution to give a D-ribo phytosphingosine derivative. The 4,5-trans double bond is introduced by coupling a p-nitrobenzenesulfonyl moiety to the 4-hydroxyl group of the D-ribo phytosphingosine derivative, and subjecting this coupled compound to a base induced elimination reaction, which favors the formation of the 4,5 sphingosine double bond. Two further reaction steps finally gave the desired D-erythro-sphingosine.
Phytosphingosine, another sphingoid base, in contrast to sphingosine, is commercially available on an industrial scale. A biotechnological process for the production of phytosphingosine by mutant yeast strains is disclosed, for example, in EP-A-726.960.